This disclosed method and apparatus relates to wireless communications systems, and more specifically to a selection mechanism for signal combining methods.
In a conventional wireless communications system, a mobile station (cellular telephone, portable computer, etc.) is served by a network of base stations. Such base stations serve as the communication relay station for mobile stations. Accordingly, a mobile station must be in wireless communication with at least one base station whenever the mobile station is turned on in order to communicate with the other components of a communications system. Mobile stations sometimes move out of a region served by one base station and into a region served by another base station. Base stations note this fact, and xe2x80x9chand-offxe2x80x9d communication from the first base station to the second. It is common for a mobile station to be in communication with both the first and second base station for periods of time. A mobile station that is in communication with more than one base station is said to be in xe2x80x9csoft handoff.xe2x80x9d In some cases, a mobile station will be in soft handoff with more than two base stations at any one time.
Soft handoff is desirable because it reduces dropped calls. In addition, soft handoff allows the mobile unit to receive the same information from more than one source and to use all of this received information (or energy) to assist in decoding the information that is being transmitted to the mobile station by each base station. Using information transmitted from more than one base station means that the power level that is required from any one base station is reduced.
One type of wireless communication system is known as Code Division Multiple Access (CDMA). CDMA systems offer greater capacity than other systems. That is, the number of channels of information that can be communicated concurrently is greater in CDMA systems than in other systems, such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system.
In CDMA systems in which both voice and data are being communicated, base stations transmit to as many mobile stations as are in the coverage area for that base station on the same frequency at the same time. In addition, each such base station transmits at the same frequency as every other base station in the network. Signals transmitted to a particular mobile station are distinguishable from signals transmitted to other mobile stations only by the fact that they are transmitted using different codes. In contrast, in a TDMA system, transmissions to a first mobile station are sent during a first period of time and transmissions to a second mobile station are sent during a second, non-overlapping period of time. In an FDMA system, transmissions to a first mobile station are transmitted on a first frequency and transmissions to a second mobile station are transmitted on a second frequency. Because a CDMA receiver can receive more than one channel at a time while tuned to a single frequency, a CDMA receiver can more conveniently perform soft handoff then can a TDMA receiver or an FDMA receiver.
While CDMA systems have the advantage of being ideally suited for soft handoff, signals transmitted to a first mobile station using a first code appear as noise to a second mobile station attempting to receive signals transmitted to the second mobile station using a second code. This interference is preferably minimized by making the codes assigned to signals transmitted from a base station orthogonal with codes assigned to all other signals being transmitted by that base station. However, codes used with signals that are transmitted by a first base station cannot be made orthogonal with codes used with signals transmitted by a second base station. Therefore, base stations carefully regulate the amount of power used to transmit signals to mobile stations. The power must be high enough to get the signal through, but the power is preferably no higher than necessary, since additional power appears to other mobile stations as additional interference and reduces the number of mobile stations that can be served.
Because conventional CDMA communication systems must handle both voice and data, certain performance requirements must be meet. One such requirement is that the delay between the time the information is transmitted from one end of the communication system until the time the information is received at the other end of the communication system must be relatively short. That is, when two people are talking, any perceptible delay between the time words are spoken at one end of the line and the time those words are heard at the other end of the line would be annoying to both the speaker and the listener.
In contrast, many data communication systems can tolerate relatively long delays between the time information is sent and that information is received. CDMA systems have recently been proposed that take advantage of the fact that relatively long delays can be tolerated in systems designed to handle only data. Such systems are referred to herein as high data rate (HDR) systems. In HDR systems, a base station is dedicated to communicating with only one mobile station at any one time. The capacity advantages of CDMA are realized with HDR systems. However, it may be difficult or undesirable to perform soft handoff due to the following reasons. First, the transmissions from a base station in an HDR system are all directed to one mobile station at any particular time. Therefore, while the number of code channels being transmitted from an HDR base station is essentially the same, all of the code channels are intended to be received by one mobile station at any one time. As a result, it is complex to coordinate the times of transmission between two base stations in order to allow soft handoff between the two base stations. Second, in order to perform soft handoff, it is necessary to distribute the same data between more than one base station. This will greatly increase the amount of data to be transferred between base stations, especially for high data rate applications. Third, system capacity increases if the mobile unit can always connect to the best serving base station instead of using soft handoff, assuming that the channel condition is relatively static, as is likely for an HDR system. This is true, since an HDR base station typically transmits at maximum power to allow the best data rate. That is, the rate at which data can be transmitted is directly proportional to the amount of power received. Therefore, in order to maximize the data rate, the maximum power is transmitted. However, this increases the amount of interference a first base station contributes to the signals received by mobile stations attempting to receive signals from a second base station.
Accordingly, there is a need for a method and apparatus for reducing the amount of interference that a first base station contributes to mobile stations attempting to receive signals from one or more other base stations.
Additionally, if the interference is omnidirectional, such as noise, an optimal signal combining algorithm may degrade receiver performance due to the existence of estimation error. Thus, there is also a need for a selection mechanism for signal combining methods that depends on interference characteristics.
The presently disclosed method and apparatus reduces the amount of interference imposed upon a first base station by transmissions of other base stations within the same communication system. The presently disclosed method and apparatus takes into account the fact that soft handoff is not easily implemented or undesirable in some communication systems, such as a high data rate (HDR) communication system in which multiple code channels are being transmitted to one receiving station, such as a mobile station, at a time. That is, in a typical HDR system, each base station is transmitting to only one receiving station at a time. To coordinate the times of transmission and data transfer between two base stations in order to allow soft handoff between two base stations is complex. Moreover, the capacity of the HDR system may increase by not using soft handoff under typical channel conditions. Therefore, the presently disclosed method and apparatus deviates from the traditional approach of performing soft handoff and relies on techniques that use two or more antennas for decreasing the interference between transmissions from a first base station and transmissions from one or more other base stations.
In accordance with the presently disclosed method and apparatus, two antennas are used to receive transmissions within a receiving station. A rake receiver is coupled to each antenna. The rake receiver has a plurality of fingers, each finger having the ability to identify and independently decode signals arriving with different propagation delays (i.e., the delay encountered between the time the signal is transmitted and the time the signal is received). By optimally combining the signals that are received by each independent finger of the rake receiver, interference that is correlated between a finger associated with the first antenna and a finger associated with the second antenna can be minimized with respect to the desired signal. Optimum combining requires determination of optimum combining coefficients as follows.
Optimum combining coefficients for each of the signals received by the fingers of the rake receiver are determined by first pairing the output from a first finger associated with the first antenna with the output from a second finger associated with the second antenna. The first finger receives the desired signal with essentially the same propagation delay as the second finger. That is, the path of the signal decoded by the first finger and the path of the signal decoded by the second finger differ only because the first finger is associated with the first antenna and the second finger is associated with the second antenna. An autocorrelation matrix is estimated. In one of the presently disclosed methods and apparatus, the autocorrelation matrix is an estimate of the autocorrelation of the received signals. Alternatively, the autocorrelation matrix is an estimate of the autocorrelation of the received noise plus interference.
In addition, the cross-correlation between the received signal and the transmitted symbol is estimated by estimating the elements of a fading coefficient vector. Each element of the fading coefficient vector is a fading coefficient associated with one of the signal paths traversed by the signals received by the rake receiver. The fading coefficient vector is preferably estimated based on the pilot bursts received on each finger.
The autocorrelation matrix of the noise plus the interference is estimated from the received noise component of each of the signals received by the fingers of the rake receiver. The received noise component for a particular finger is calculated by subtracting the fading coefficient associated with the signal received by that finger from the total signal received by that finger in the pilot bursts. In another of the presently disclosed methods and apparatus, the noise and interference is estimated by subtracting the signal y(m) from the signal y(m+1) one chip later in time (i.e., subtracting adjacent samples). In yet another of the presently disclosed methods and apparatus, the autocorrelation matrix of the noise plus interference Rnn is estimated by subtracting the fading coefficient vector multiplied by the transpose conjugate of the fading coefficient vector, from the autocorrelation matrix Ryy of the received signal y(m).
Once the fading coefficient vector and the autocorrelation matrix of the signal received by each pair of fingers has been calculated, these values are used to calculate the optimum combining coefficients. Alternatively, once the fading coefficient vectors and the autocorrelation matrix of the noise plus interference for each pair of fingers have been calculated, they are used to calculate the optimum combining coefficients.
Upon combining the signals received by each finger of the rake receiver using the optimum combining coefficients, it is desirable to calculate the signal to interference plus noise ratio of the output from the optimum combiner. This ratio is calculated using the transpose conjugate of the optimum combining coefficients and the fading coefficient vector. The result is a system in which interference from sources other than the source of the desired signal is suppressed with respect to the desired signal to improve decoding. The resulting signal to noise plus interference is calculated to allow a receiving station so equipped to determine the data rate that the channel can support.
In another aspect, the present invention is directed to a selection mechanism for signal combining methods that depends on interference characteristics. Accordingly, in one aspect of the invention, a method of computing optimal combining coefficients for a plurality of received signals advantageously comprises the steps of estimating an autocorrelation matrix for the plurality of signals based on received noise and interference associated with the plurality of signals; comparing the magnitudes of off-diagonal elements of the autocorrelation matrix with the magnitudes of diagonal elements of the autocorrelation matrix; setting the off-diagonal elements equal to zero if the magnitudes of the off-diagonal elements are significantly less than the magnitudes of the diagonal elements; and computing a plurality of optimal combining coefficients based on the autocorrelation matrix.